Biomedical Engineering Reference
In-Depth Information
5 rotations. Consequently, the bias variation
Γ
is 225 nm per screw turn and the total bias
range
Γ
tot
is 1125 nm.
As described in
Section 2.1
, the Senarmont technique produces bias, which is linearly
proportional to the analyzer rotation. Using
formulae (2.7) and (2.5)
we can find
dependence of the bias
Γ
on the analyzer orientation angle
χ
:
χ
180
λ
Γ
5
(2.18)
Rotation of the analyzer from
2
90
to 90
introduces bias from
2
λ
/2. The total bias
range equals to one wavelength. The high bias accuracy is one of the advantages of the
Senarmont method. For example, measuring the analyzer orientation with precision 0.1
will give the bias accuracy of 0.3 nm at wavelength 550 nm.
/2 to
λ
The liquid crystal variable retarder creates a bias, which depends on the applied voltage.
However, this dependence is not linear. Therefore, it is necessary to have a calibration
curve. The thinner liquid crystal variable retarders are faster and provide the bias change
less than one wavelength. The thicker liquid crystals could give the bias change about two
wavelengths. But they are considerably slower and more sensitive to the ambient
temperature variation.
In order to create a large bias, it is possible to insert in the optical path an additional full-
wave plate, which is also called the unit retardation plate or red plate
[26]
. This birefringent
plate is cut of such a thickness as to have a retardation of one wavelength of yellowish
green (550 nm), thus giving the sensitive first-order red between crossed polars in white
light. Slight changes in the optical path difference by addition or subtraction of path
differences by the specimen over which the plate is placed are very noticeable, and
accordingly the plate is called a sensitive tint plate
[39]
. The color variation can be used for
estimation of optical path difference of the specimen.
Figure 2.5
shows DIC images of 7
m diameter glass rod immersed in Fisher Permount
mounting medium (Fisher Scientific,
http://www.fishersci.com
). The refractive indices of the
glass rod and the Permount at wavelength 546 nm were 1.554 and 1.524, respectively. The
pictures were taken in white light using an Olympus BX-61 upright microscope equipped
with the high-resolution DIC prism U-DICTHR, silicon oil objective lens UplanSApo30
3
/
Hamamatsu 3CCD Cooled Digital Color Camera ORCA-3CCD (Hamamatsu Photonics,
http://www.hamamatsu.com
). The shear plane is oriented in the northwest direction.
μ
All images were captured with same exposure time, 0.1 s.
Figure 2.5A
represents a case
when the bias is zero.
Figure 2.5B
was taken with bias of 100 nm (one full rotation of the
translation screw). As one can see, the right bottom side of the rod is black. This means that
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